Optical tomography for flow field diagnostics

Development and validation of a reconstruction algorithm for three-dimensional nonlinear tomography problems

“…Figure 2(a) shows the setup of the validation experiment from the top view. The setup used a cubical cell, with a dimension of 50 × 50 × 50 mm 3 , filled with a fluorescence dye solution (a well stirred mixture of ethanol and Rhodamine 6G) to experimentally create a uniform phantom. When illuminated volumetrically by a laser at a wavelength of 532 nm (generated by a pulsed Nd:YAG and expanded by a series of lenses as shown), the dye solution absorbs the illumination laser as it propagates through the solution and emits LIF photons, resulting in a nonlinear problem as described above.…”

“…In this volumetric LIF problem, the two-level LIF model was used and the fluorescence yield was set as a constant value for all particles in the concentration field (i.e., the goal is essentially to measure a relative concentration field, which is what many LIF measurements are aimed at). The computational domain was taken to be a volume of 40 × 40 × 40 mm 3 center around the origin, smaller than the volume of the dye cell to exclude non-ideal effect around the edges and corners of the cell. The computational domain was then discretized into 120 × 120 × 120 voxels (i.e., N ≈1.8 × 10 6 as shown in Eq.…”

“…The goal of tomography is to reconstruct a spatially-resolved distribution of a certain property of a target based on a set of line-of-sight integrated measurements of the target measured from different perspectives (and these measurements are called projections). Different applications involve different target and physical processes, and examples range from the medical imaging of human body using optical transmission measurements [1], examination of the internal structure of solids or liquids using electrical capacitance measurements [2], and optical imaging of a chemical species in fluid flows using absorption or emission measurements [3,4]. Regardless of such wide range of applications and the corresponding physical processes involved, the mathematical background involved remains essentially the same.…”

Mapping tracer gas concentrations using a modified Low Third Derivative method: numerical study

“…When a limited number of beams from a few projection angles are used, artefacts such as streaking and distortion can be encountered in the reconstructed gas concentration map (Santoro & Semerjian, 1981). These reconstruction errors that arise on account of insufficiency of data are usually called aliasing distortions.…”

Tomographic imaging of fluid flows by the use of two-tone frequency-modulation spectroscopy

“…The experimental technique is described in detail in a previous paper.' 3 In this Letter we describe the application of an optical dual-channel technique with balanced homodyne detection.1 4 This technique can be employed to suppress optical interference fringes, residual amplitude modulation, and laser excess noise in order to reach an ultrasensitive detection limit. In our approach the balanced detection scheme effectively cancels the background absorption that is due to atmospheric 02.…”